North American Signaling:

IC Cab Signaled APB without Intermediate Block Signals

In 1922 the Interstate Commerce Commission ordered a number of Class 1 railroads to install train stop or train control equipment on at least 2 crew districts. The order was an attempt to accelerate the development and application of automatic train stop systems. The Illinois Central decided to test the use of cab signals and automatic train stop to eliminate most wayside signals. The test was conducted on a double track line and on the single track line between West Waterloo to Ft. Dodge, Iowa (the Waterloo District within the Iowa Division). The latter installation - supplied by the Union Switch and Signal Co. and commissioned in the mid 1920s - is the subject of this document. Thanks to Jon Roma for providing input.

Before reading further, it is recommended to become familiar with the method of operating trains on single track, the "tumble-down" and the protection of opposing trains around sidings on the standard Absolute Permissive Block (APB) signaling.

The Illinois Central Cab Signal System

The cab signal system that IC adopted was developed and manufactured by Union Switch & Signal Co. This particular system is probably the simplest of all the cab signal systems in that it only carries two different cab signal indications to the engineer. The system architecture is, however, basically the same as in later and more advanced systems from US&S.

The main principle of the cab signal system is that an pickup coil ("antenna") located in front of the train's leading axle can pick up an alternating current (AC) in the track circuit. The first axle of the train short circuits the rails and thus the coil needs to be ahead. In case of the IC system the AC is superimposed on a direct current (DC) track circuit. When the train's pickup coil detects the AC in the rails it is interpreted as a "Clear" signal (green, proceed) and when it does not detect AC the cab signal drops to "Restricting" (red, immediately reduce to Restricted Speed).

The cab signal indication is shown as a red or green light in the cab. A whistle in the cab announces whenever the cab signal indication changes. A short blast if the signal changes from red to green and a long blast if the signal changes from green to red. If the cab signal changes from green to red the engineer must acknowledge this via a lever in the cab. If acknowledgement is not given within 6 seconds the brakes are automatically applied. After a brake application the system must be reset via a valve located under the cab so that the it cannot be reset before the train is brought to a stop.

In conventional signaling a train will see a sequence of first a green, then yellow and finally a red signal when approaching an occupied block. The train continues until the yellow, then slows down prepared to stop at the next (red) signal. Obviously assuming the signals are placed at least a braking distance apart.

If cab signaling is added the AC will be sent out into the blocks approaching all signals that allows the train to pass at speed. As can be seen there is no useful information in the lineside signals that that the cab signal doesn't give and thus the lineside signals can be done away with.

The next lines show the similar situation for a train with a cab signal. The train continues running as long as the cab signal shows the "Clear" indication.

When the train enters the last block before the occupied block the cab signal drops to "Restricting", the whistle sounds, the engineer acknowledges the whistle and starts braking the train to Restricted Speed.

As previously mentioned the lineside signals no longer carry any necessary information - it is all in the cab signal. The exception from this is an absolute signal. The most restrictive cab signal indication still allows the train to continue at Restricted Speed and something additional is required for a "Stop and Stay" indication. The solution to this was to keep the absolute signals but only as red/green signal with red meaning "Stop" (and stay) and green meaning "Proceed" (according to cab signal). In case of APB ths would be the case for the Headblock signals.

The locations where the intermediate signals would have been placed in a conventional signal system were dubbed "Phantom Locations" as the crews would still typically experience the downward changes in cab signal indications at these locations and the equipment cabinets would be marked with identification like a conventional block signal location.

The cab signal AC needs to be fed into the arrival end of the track circuit in order for the train to read it. Otherwise the train will shunt the AC. On bidirectional tracks this means that the feed must be switchable from one end to another, depending on the train's direction of travel.

On single track ABS/APB systems, however, there is no controlled direction of traffic that can be used to direct the AC feed. But any approaching train is protected by signals at red. So the rather elegant solution to controlling the cab signal AC is to only transmit it away from green signals if the opposing signal is red at the same time. A convenient side effect is that a signal system failure that leaves an opposing signal green will result in no cab signal AC and thus be exposed.

On single track ABS/APB systems, however, there is no controlled direction of traffic that can be used to direct the AC feed.

At each signal location is a short wire loop between the rails, extending 50' to each side of the location, transmitting a permanent Cab Signal AC. These are the so called Acknowledgement Loops and their purpose is to re-engage the cab signal of a train running on "Restricting" by briefly changing to "Clear" and then drop back to "Restricting". This requires the engineer to once again acknowledge the "Restricting" indication. For trains passing a signal location on cab signal "Clear" the loops have no effect.

The Illinois Central Cab Signal APB

The illustration above shows a standard configuration of a section of IC Cab Signal APB line. To the left is one end of a siding with its Headblock Signal and the opposing "Entering" phantom signal. Then follows a section of intermediate phantom signals where intermediate permissive block signals would have been on ordinary APB. Then another siding and a piece of the line continuing towards the right. As with standard APB the signals are green when the block system is at rest (i.e. no nearby trains on the main track and the main track switches lined normal). As explained above the all green signal locations do not transmit cab signal AC. The Acknowledgement Loops are omitted from these examples for clarity.

Lets follow a train across this section of line. First, the train is approaching from the left. For now it has not progressed past the siding switch at the left siding and no tumble-down has yet taken place. With the line between sidings all clear there is no cab signaling active yet but the train sees a "Clear" cab signal indication coming from the headblock signal:

As soon as the train passes the "Headblock Signal" the tumble-down is triggered. All the phantom signal locations until next siding now have a direction of travel and start transmitting "Cab Signal Clear" to the train. Note that on the rightmost siding both the opposing Headblock Signal and the "Entering" phantom signal on approach to the siding go red. The latter provides protection similar to the double yellow protection that ordinary APB provides in this situation. Also note that the train receives "Cab Signal Clear" from the rear but of course this has no effect on that train:

As the train moves across the line, the signals clear up behind it (as
on ordinary APB):

For the sake of the example a leftbound train now tumbles down the line towards the rightmost siding. Now both Headblock Signals and both Entering phantom signals are red. With both signals at both ends red, no "Cab Signal Clear" is transmitted but the train still runs on "Cab Signal Clear" from the last intermediate phantom signal:

When the rightbound train passes the last intermediate phantom signal location it no longer receives "Cab Signal Clear". The cab signal drops to red and requires acknowledgement from the engineer and the train to slow down to Restricted Speed. This situation is equivalent to a train seeing a yellow on the last intermediate signal before a siding:

Our rightbound train stops and opens the switch to enter the siding. Still with a red cab signal. Opening the switch shunts the track circuit the same way a train would, i.e. as far as the block system is concerned the block is occupied whether from a train or an open switch:

When the train is in the siding (and off the track circuit) the switch shunt prevents the leftbound train from picking up the "Cab Signal Clear" that is being transmitted from the now clear leftbound Headblock Signal:

Once the switch is back to normal the leftbound train gets a green in the cab signal right away. On regular APB the train would have had to run the entire block at Restricted Speed until it passed the clear Headblock Signal but sith cab signaling the train can be upgraded immediately:

Again for the same of the example, another leftbound train tumbles down the line, making this a double meet:

The 2nd leftbound train catches "Restricting" when approaching the station while the 1st leftbound has a "Cab Signal Clear":

As soon as the 1st leftbound clears the first block section the Headblock Signal behind it goes "Clear", lifting the "double red" that is restricting the 2nd train:

If the 2nd leftbound train is catching up on the first it will pass the clear Headblock Signal and then get "Restricting" from its cab signal (on regular APB the Headblock Signal would have been yellow):

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